SDL input

Tested on all platforms with SDL2 rc snapshot ( http://www.libsdl.org/tmp/download-2.0.php )
New features:
Hardware mouse cursors - http://bugs.openmw.org/issues/263
Allow mouse outside of window while paused - http://bugs.openmw.org/issues/449
openmw.png used as window icon
Fixed alt-tab, multimedia keys etc on linux